Radar markers



April 30, 1968 W. B. TARPLEY, .1R

RADAR MARKERS Filed Aug. 24. 1966 [IHUHUHHE l/VVE/VTOR W/LL/M E.MRPLE'Y. JR.

@y MM ATTORNEYS.

United States Patent 3,381,293 RADAR MARKERS William B. Tarpley, Jr.,West Chester, Pa., assignor to Aeroprojects Incorporated, West Chester,Pa., a corporation of Pennsylvania Filed Aug. 24, 1966, Ser. No. 574,794Claims. (Cl. 343-18) ABSTRACT 0F THE DISCLOSURE A radar marker for freefalling .air deployment comprising a tab of flexible radar transparentmaterial, a weight disposed at the bottom portion of the tab, and aplurality of radar reflective members disposed on the tab.

This invention relates to radar markers, and more particularly, radarmarkers of the type comprising an array of metallic needles.

A long felt problem has existed in connection with the use of radarmarking needles. Such needles can be very but are usually made muchthicker for ease of fabrication, handling, and dissemination. It hasbeen found that by deploying a large enough number of such needles, acloud having a radar cross section sufllcient for detection by ordinaryradars may ibe obtained. The |use of such needles, however, presentsserious difllculties due to their small size and light weight. Thus,dispersion of the needles results in a rapid decrease in the crosssection of the marker, and accordingly, rapid fading of their radarindication. The fading problem is greatly increased when the needles aredeployed in rapidly moving or turbulent air, such as is encountered inthe slipstream of an aircraft or under storm conditions, and in heavyprecipitation.

The present invention is a radar marker in which meta-llic elementscorresponding to radar marker needles are disposed on backing strips.Thus, the orientation of the needles one to the other is fixed at thetime of manufacture, and at least insofar as each individual strip isconcerned, the radar cross section is constant. Furthermore, apreordered array of radar reflectors can be so designed as to have aconsiderably augmented return compared to a random array as found in adisposed cloud of needles. Moreover, the strip is designed to assume adesired orientation when deployed. Thus, in a desired embodiment of theinvention, the backing strip is a flexible plastic member and aweighting material, such as a layer of metallic material, is disposed onthe bottom of the strip. When such strips are released in the air,gravity acting on the weighting material causes such material to actmuch in the same way as the tail of a kite, thereby giving all of aseries of strips a common vertical orientation.

The present invention comprehends deployment of the thin, such as on theorder of 3() micro-inches in thickness novel radar marker bydisseminators such as those disclosed in copending application Ser. No.572,473, led Aug. 15, 1966 assigned to the assignee of the presentinvention. Other disseminators could also be used.

Accordingly, it is an object of the present invention to provide a novelradar marker.

It is another object of the present invention to provide a novel radarmarker wherein .marker needles are permanently oriented with respect toeach other.

It is still another object of the present invention to provide a radarmarker having a longer lasting indication than needle markers heretoforeknown.

KAIt is still another object of the present invention to provide a radarmarker which can be rapidly and easily deployed.

Other objects will appear hereinafter.

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For the purpose of illustrating the invention there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIGURE 1 is an elevation view of a radar marker in accordance with thepresent invention.

FIGURE 2 is an enlarged cross-sectional view taken along the line 2-2 inFIG-URE 1.

Referring to the drawings in detail wherein like numerals indicate likeelements, there is seen in FIGURE 1 a radar marker element designatedgenerally by the reference number 10. The element 10 comprises a backingstrip 12 and a series of needles 14, 16 and 18 thereon. Backing strip 12is ideally made of flexible radar transparent plastic material. Thepolyethylene terephthalate lm sold by E. I. du Pont de Nemours & Co.,Inc. under the trademark Mylar is especially suitable. The presentinvention cornprehends a backing strip of approximately one-half milthickness, and approximately two inches square. It has Ibeen found thatsuch a backing strip provides ample area for the disposition of aneffective array of marker needles, yet is small enough and flexibleenough to be easily ejected from disseminators.

The needles 14, 16 and 18 are disposed in rows across the face of thebacking strip 12. The effectiveness of such needles as radar reflectorsis determined vat least in part by their length and the length of theincident radar wave. For example, in X band radar, whose wave length is5.8 to 2.7 centimeters, the best needle length would be from 2.9 to l.4centimeters. Also, the effectiveness of the needles as reflectors isdetermined in part by their spacing one to the other. Such spacingshould be on the order of one wave length (of the signal to bereflected) apart or less. It is also important that the needles beperfectly aligned.

Referring to FIGURES 1 and 2, it is evident that needles 14, 16 and 18are of different dimensions, and are differently spaced. Thus, theillustrated radar marker is a composite marker, capable of serving as aneffective reflector for radar waves of three carrier frequencies. Forexample, in usage, the needles 14 may be of such length as to optimizetheir reflections of X lband signals, the needles 16 may be adapted forK band reflection, and the needles 18 for Q band. It will beappreciated, therefore, a radar marker in accordance with the presentinvention may be tailored so that its reflective properties are bestsuited to its intended usage. Thus, a marker intended for use in .asurvival pack carried by aircraft on over water flights would ideallyhave needle arrays corresponding to common types of marine and airborneradar. Referring again to FIGURE l, it has been found that the spacingbetween the ends of the needles of adjacent rows, for example, thespacing between the end of the needles 14, and 16, is not critical.Thus, such spacing need only be so much that there is a non-conductivegap.

In one form of the invention, it is contemplated that the needles bealuminum, which may be deposited on the backing strip 12 by metalizingapparatus such as is disclosed in the U.S. Patents 2,622,041 and2,665,224. Other metals, of course, could be used. When so applied, theneedles are merely metalized portions of the strip, and in no way impedefolding or rolling of the strip for packing in a disseminator.

Referring again to FIGURES 1 and 2, the invention further comprehendsthe use of a weight 2t) secured to a bottom portion of the backing strip12. Such weight ser-Ves much the same function as the tail of a kite,and serves to stabilize the strip in a vertical orientation when it isdeployed in the atmosphere. Such an orientation is effective to maximizethe target presented by the needles. A suitable weight 20 comprises aband of relatively heavy material, such as ferric oxide, applied to thebacking strip 12 by the techniques set forth in the above U.S. patents.It should be understood, however, that the weight 20 may comprise othermaterials, and may be applied in any desired manner. The weight 20 isnot intended to substantially aiect the radar reective properties of theradar marker.

In usage a large number of radar marker elements 10 may be disseminatedfrom apparatus such as that of the above-identified co-pendingapplication, or projected by other well-l nown means. The elements 10,due to the effeet of the Weight 20, assume a vertical disposition asthey fall. Such disposition serves to maximize the effectiveness of themarker. Also, as explained heretofore, the Xed orientation of theneedles with respect to the backing strip minimizes the eiect ofdispersion due to wind and atmospheric conditions. The overall result isa marker far more eiective for its intended purpose than thosepreviously known.

The present invention may be embodied in other specific forms Withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specication as indicating the scope oftheinvention.

l claim:

1. A radar marker for free falling air deployment comprising a tabformed of a exible radar transparent backing tab, a weight disposed on abottom portion of said tab to cause said tab to assume a generallyvertical orientation when said tab is permitted to fall through theatmosphere, a plurality of conductors disposed on said strip, saidconductors being generally parallel elongated needles arranged in aplurality of spaced parallel rows, the needles of each row being ofequal lengths and the respective rows having needles of diierentlengths, the needles of the respective rows being spaced apart bydifferent distances equal to or less than one wave length of a radarsignal to be reflected from said row, and the lengths of said needles ofeach row being determined by the wave length of the signal to bereflected from said row so that said radar marker is adapted to reflectsignals of different wave lengths.

2. A radar marker in accordance with claim 1 wherein said backing striphas a thickness in the range of .00025 to .0005 inch.

3. A radar marker in accordance with claim 1 wherein said weightcomprises a layer of material overlying said backing strip.

4. A radar marker in accordance with claim 1 wherein said needles aremetallic lms overlying the surface of said strip, said films having athickness of up to 30 microinches.

5. A radar marker in accordance with claim 4 wherein said thickness isabout 30 micro-inches.

References Cited UNITED STATES PATENTS 2,758,203 8/1956 Harris.2,840,819 6/ 1958 McClellan. 2,881,425 4/1959 Gregory 343-18 3,122,7432/1964 Vlasic 343--18 3,206,749 9/1965 Chatelain 343-18 RODNEY D.BENNETT, Primary Examiner.

B. L. RIBANDO, Assistant Examiner.

